The present invention relates to novel heat-absorbing fusion glasses for the encapsulation of electrotechnical components.
Fusion glasses having a specific IR absorption due to doping with FeO and which can be processed hot by mere radiant heating with quartz-iodine lamps, are preferably used for the hermetic encapsulation of magnetically operated contacts (reed switches) and have been part of the state of the art for more than a decade.
The metal alloys employed (determined largely by the properties required for the end use) and the techniques and to manufacture such reed switches have resulted, in the last decade, in a series of diverse requirements on the properties of fusion glasses. From these demands there have evolved a group of reed glasses which, in some aspects, are very different.
Among these requirements was primarily the avoidance of lead oxide which vaporizes most readily at normal processing temperatures of about 1000.degree. C. and quickly changes the effective radiation transmittance by its condensation on windows and mirrors. Further required was the minimizing of other readily vaporizable components, such as K.sub.2 O, B.sub.2 O.sub.3 and F.sub.2, which can lead to troublesome condensates in the switch chamber.
A novel way to solve the problem posed by these requirements was provided by the development of glasses having an extremely low softening point (German Pat. No. 2,503,793), whereby the vaporization of glass components such as K.sub.2 O, PbO, B.sub.2 O.sub.3 and F.sub.2 could be suppressed adequately by use of correspondingly low processing temperatures. Furthermore, these low softening point glasses provided the opportunity for use of magnetic alloys (semihard alloys) in reed switches. The magnetic properties of these alloys are critically dependent on their heating history, so that they would be unduly altered at the fusion temperatures required by normal reed glasses.
However, these developments and improvements of reed glasses have not yet provided a solution to the problem of the crack sensitivity of these glasses. Such cracks can arise during wire sealing due to excess stresses during cooling, during the tin-plating of the wires and during exposure of the switch to alternating temperature stresses while in use. These cracks, further, frequently represent a significant waste factor during the manufacture of reed switches. The tensile stresses which cause such cracks result from the difference in the degree of contraction of metal and glass. For the metal alloys customarily used in reed switches, these stresses have a temperature dependence illustrated by the curve of FIG. 1.
If the room temperature (RT) stress caused by the desired and necessary radial forces as well as the permissible glass stresses is fixed by the selection of the glass-metal partners, then, for a given fusion geometry and a fixed cooling rate, the curve of the stress in dependence on the temperature is likewise predetermined.
The level of maximum tensile stress and the values of the tangential tensile stress occurring during the subsequent operating temperature govern the crack sensitivity of the wire seals during cooling and during use. Depending upon the desired or necessary radial compressive strain involved in the operating range, corresponding axial and tangential tensile stresses in the glass occur in this range.
Even if the wire sealing step withstands the shortterm stress of .sigma..sub.Z, max (FIG. 1), for example during rapid cooling in air, nevertheless, cracks can still occur in the glass due to the great dependency of the glass strength on the longer term stresses occuring during relatively long or repeated stays at a given temperature &lt;T(.sigma..sub.max). Besides the temperatures of the normal use of reed switches, such increased and dangerous temperature loads occur, for example, in cleansing baths and during the tin-coating of the wires.
If wire fusion seals are to have crack resistance under such temperature loads, it is necessary to provide a low value for .sigma..sub.Z, max and/or a low steepness of the polarimeter curve between RT and T (.sigma..sub.Z,max). (See FIG. 2).
A prerequisite for this is that the glass solidify during cooling at a time which corresponds as closely as possible to that of the quasilinear portion of the expansion curve of the metal (the small intersecting angle between the contraction curves). In this way, the maximum contraction difference after reaching the glass point, influenced by the above-described maximum stress, will remain as small as possible (curve 2 in FIG. 2).
The position of this transition temperature T.sub.G (a fictitious variable which is used as a substitute means for describing a practically constantly progressing solidification process by viscosity increase) in relation to the contraction curve of the glass in the transformation range is caused by relaxation mechanisms extensively dependent on the glass composition. Accordingly, T.sub.G also depends on the cooling rate, and, thus, the contraction difference in glass-metal fusions and the associated temperature dependent stress curve also prove to be dependent on the cooling rate (see FIG. 3).